The present invention relates to methods and apparatus for inspecting wafers.
In particular, the invention relates to apparatus disposed in single and multi-chamber cluster tools for inspecting wafers.
During conventional fabrication of semiconductor wafers, cluster tools transport the wafer between various stations, such as, for example, a chemical vapor deposition station or an etching station. After fabrication, the wafer is transported from the cluster tool to an inspection station and inspected for surface defects, line width, electrical functions, and the like.
Generally, wafers are not individually inspected because of the disparity between the throughputs of the fabrication machinery and the inspection machinery. Individual inspection for each wafer would either require a prohibitive amount of inspection machinery to maintain adequate throughput, or result in an unacceptable loss of productivity. Accordingly, wafers are sampled for inspection, with the sampling rate and selection method being based on the process involved.
Empirically, manufacturers know that certain processes are more stable than others, and select the sampling rates for each process accordingly. For example, some processes are very stable and, once the process is adjusted to produce parameters that are within the inspection criteria, the parameters do not vary greatly over time. In these cases, once the process is adjusted, the processing machinery can operate relatively autonomously for days at a time. Thus, stable processes do not require a high sampling rate. Other, less stable, processes require a higher sampling rate.
Generally, wafers are processed in lots of 20 to 25 wafers each, and usually with 4 to 5 lots processed between cleanings of the processing chambers. In the case of 300 mm wafers, the lot size is about 12-13 wafers. With a low sampling rate as used with more stable processes, it is possible for many wafers to complete the process having defects. For example, in an otherwise stable process, the process chamber may suffer an excursion, such as a blown o-ring or electrical arcing.
Accumulated process material, such as etchant or deposition material, may flake off the walls of the chamber onto the wafers. If the excursion occurs early in the first lot or shortly after a sampling, for example, the low sampling rate can produce enormous waste in terms of the number of defective wafers that consume processing time and material before the problem with the process is detected during the next sampling. A higher sampling rate could minimize this problem, but, as noted, productivity would suffer as a consequence.
Defects resulting from such casualties to the process chamber result in large scale defects, on the order of 0.5 micron in size. In the past, manufacturers have not inspected separately for such large defects because large defects are discovered during inspection for smaller defects. Yet these large scale defects account for a large proportion of defective wafers.
The smaller defects are typically caused by instabilities in the various processes, and the instabilities are factored into the sampling rate to minimize the number of defective wafers that go through processing before a sampling detects the problem. The larger defects, on the other hand, are generally unpredictable, being caused by a catastrophic breakdown, and can therefore cause the greatest loss in terms of waste.
Manufacturers are striving to detect ever smaller defects, such as 0.15 to 0.18 micron-sized defects. Unfortunately, the equipment necessary to detect these smaller defects is very large, expensive, complicated, and takes up a lot of valuable floor space.
In particular, as the detectable defect size shrinks, the corresponding inspection machinery increases in size, complexity, and cost. For example, in order to determine line width in the 0.15 to 0.18 Micron range, inspection machines require very large granite or marble tables to provide a stable, non-moving platform on which to perform the inspection. These tables are quite large and have a large footprint, taking up valuable manufacturing floor space. In addition, the large inspection machines have a reduced throughput. The reduced throughput requires a lower sampling rate which results in higher waste or lower productivity.
Moreover, as the wafer size increases towards the 300 millimeter size, the handling equipment necessary to move the wafers around also increases in size and complexity, with a resultant slowdown in handling speeds.
Thus, manufacturers would welcome a method and apparatus for a quick, real-time sampling of wafers. Quick, real-time sampling would allow a higher sampling rate while minimizing any adverse impact on throughput and result in early detection of large defects. Early detection of large defects would minimize the waste associated therewith by saving the remaining wafers in the lot from further processing, thereby saving time and material. Moreover, such real-time sampling would reduce the sampling burden on the large inspection machines or effectively increase their sampling rate.
The present invention overcomes these disadvantages and others by providing an inspection station coupled to the cluster tool. Coupling the inspection station to the cluster tool provides a method and apparatus for a quick, real-time sampling of wafers that would detect large defects sooner while minimizing any adverse impact on throughput.
According to the present invention, a semiconductor wafer inspection station comprises a cluster tool and an inspection station attached to the cluster tool. The inspection station includes an image detector for detecting an image of the semiconductor wafer, and a processor for processing the detected image to detect defects in the semiconductor wafer.
In preferred embodiments of the invention, an inspection chamber is attached to the cluster tool, and the inspection station is disposed in the inspection chamber. The inspection chamber includes a rotatable chuck and the inspection station includes a light source positioned to illuminate the semiconductor wafer when it is positioned on the chuck. An image detector is positioned for receiving light that is reflected by the semiconductor wafer and a processor is coupled to the image detector for processing the detected image to detect defects.
The present invention also provides a method of inspecting a semiconductor wafer. The method comprises the steps of providing a cluster tool, attaching an inspection station to the cluster tool, and positioning the semiconductor wafer at the inspection station for inspection. A light source illuminates the semiconductor wafer and a receiver receives a reflected image. A processor coupled to the receiver processes the image to detect defects. When the inspection detects a defect, the inspection station sends a warning to an operator. Thus, the invention provides for inspection of the semiconductor wafer before it leaves the cluster tool/inspection station.
The present invention offers several advantages, such as reducing wafer loss and providing for improved sampling without hampering the throughput of the cluster tool. For example, after a process has taken place, the wafer is passed under a glancing laser-type apparatus which is controlled by the same software that controls the cluster tool. In the event a defect is detected, the tool can either shut down or provide a warning to an operator, thereby reducing wafer loss by preventing the processing of other wafers until the problem is corrected. If no defect is detected, the wafer continues with further processing steps, as necessary. If the wafers are sampled for testing, the uninspected wafers continue through the processing as before, leaving the throughput unaffected. However, a glancing laser-type apparatus can quickly detect a 0.5 micron defect, which allows a higher sampling rate, thereby reducing waste and increasing productivity. Importantly, the invention achieves these advantages without increasing the footprint of the equipment, thereby preserving valuable floor space.
These and other features and advantages of the invention will become apparent from the following detailed description of preferred embodiments of the present invention.